When using pressure sensors, the output signals 0 ? 20 mA, 4 ? 20 mA and DC 0 ? 10 V are generally chosen in order for the sensor signals to be evaluated and further processed. Because of this, the signal output of the pressure sensor is normally connected to a corresponding input card in the PLC.
In this context it can often be confusing, as the day-to-day using the terms ?active?, ?passive?, ?current source?, ? Vicious ?, ?current sink? and ?load? tend to be wildly mixed together. Any electrical signal processing always takes a voltage supply (an ?active part?) and a ?load?, such as a pressure sensor, which represents the ?passive part?. Sometimes the active part of the interconnection is also described as a power source/voltage source and the passive part is referred to as a ?current sink?. To ensure that an electrical circuit can function, current must flow in a circuit ? even though an instrument is usually known as a load, the current isn’t consumed because of it, rather it only flows from the existing or voltage source through the load and back to the current source.
This works only when an ?energy gap? exists between current source and current sink, so the power source operates actively (= sending out current) and the current sink passively (= current flows through it) . Therefore, an interconnection of two current sources or two current sinks won’t operate normally. This situation is complicated in day-to-day application:
When does a pressure sensor work passively (current sink) so when does it work actively (current source)?
So how exactly does the input card in my PLC operate?
As a rule of thumb, you can keep in mind that 2-wire sensors usually work passively and thus need an active PLC input card. It is difficult with 4-wire sensors, since, for instance, a 4-wire flow sensor includes 2 wires for another voltage supply and 2 wires for an active or passive 0/4 ? 20 mA signal output. It is therefore vital to check the datasheets for the sensor and PLC input card used.